1. Field of the Invention
The present invention relates to an image forming apparatus having a heat fixing device and, more specifically, to an image forming apparatus in which fixing immediately after the end of warming up of the apparatus, that is, fixing in the first image forming operation in the morning, can be carried out satisfactorily.
2. Description of the Related Art
Fixing means of an image forming apparatus of particular interest to the present invention is disclosed in U.S. Pat. No. 4,385,826. According to this gazette, a heat roller method is employed for fixing transferred toner images on a sheet of copy paper. At the start of warming up of the apparatus after the power is turned on, the heat roller and the pressing roller are kept still. When the surface temperature of the heat roller reaches a prescribed temperature, both rollers are rotated for a prescribed time period being in contact with each other, whereby the heat roller and the pressing roller are both warmed to a temperature high enough for fixing. The fixing device has been developed to reduce the time required for the rising of the surface temperature of the rollers during warming up and to carry out the fixing operation satisfactorily.
Another fixing apparatus of an image forming apparatus of particular interest to the present invention is disclosed in Japanese Patent Laying-Open No. 62-143067. According to this Laid-Open Invention, when the surface temperature of the fixing roller becomes lower than a prescribed temperature during continuous copying operation, the copying speed is reduced. In this fixing apparatus, the copying operation is continued without interruption, even when the fixing temperature of the fixing apparatus becomes lower than a prescribed temperature during continuous copying operation. However, the operation at the time of power-on is not mentioned at all in the above article.
However, the above described fixing apparatuses have the following drawbacks. In the former fixing apparatus, the pressing roller as well as the heat roller are rotated from the time when the surface temperature of the heat roller reaches a prescribed temperature during warming up after the turning on of the power. When the surface temperature of the heat roller and of the pressing roller reached the aimed temperature for fixing by the heat conduction from the heat roller, the rotation of both rollers is stopped and thus the warming up is completed. Therefore, the copying operation cannot be carried out after the lapse of a prescribed time period after the turning on of the apparatus. Namely, it takes much time to start the copying operation after the turning on of the apparatus. In addition, preparatory rotation of the rollers is suddenly started during warming up of the apparatus, that is, when the copying operation is totally unexpected. Therefore, the user may possibly be surprised.
In the latter mentioned fixing apparatus, the copying operation is not interrupted even if the fixing temperature becomes low during continuous copying operation. However, no consideration is taken for the first copying operation in the morning after the end of warming up.